Dear Traveler, Everest Base Camp in September is nice and warm, it is best time to trek after monsoon in the Himalayas. You can see crystal clear mountain views. That will be nice and ward during the day time and cold at night time. You have to prepare from -1 to 17 °C. You can walk with a t-shirt and a wind breaker during daytime.
Have a nice trek.
http://www.nepalgatewaytrekking.com/ about 5 years ago